In many mammalian species, cervical-vaginal stimulation received during mating abbreviates the period of estrus or heat; recent experiments have demonstrated that, in the female rat, the temporal pattern of such stimulation contributes to the rate at which sexual receptivity declines at this time. The proposed experiments would examine the endocrine and neuroendocrine mechanisms involved in regulating the duration of estrus in the rat. This research has three main objectives: 1) to determine whether effects of temporally-patterned coital stimulation on estrus duration are associated with changes in the reproductive capacity of the individual; 2) to determine whether circulating androgens, particularly 5Alpha-reduced androgens, facilitate the decline in estrous behavior following coital stimulation; and 3) to determine whether temporally-paced coital stimulation results in an acute release of luteinizing hormone (LH) which, through stimulation of ovarian and/or adrenal androgen release, might facilitate the process of estrus termination. Results of these studies will contribute to the understanding of the process of estrus termination in the rat, and will examine whether this process influences significantly the reproductive capacity of the species.